At construction sites or where buried pipes or wires are being repaired, it is frequently necessary to dig trenches or holes which are worked in by construction personnel. Typically, when a utility company is laying or repairing buried pipes or wires, such trenches will be worked in over a number of days. The trenches or holes are generally covered with steel plates when not in use by workmen so that vehicle and pedestrian traffic can proceed without danger.
The plates used to cover such trenches or holes are generally made of steel and are about one inch thick, about 5-7 feet wide, and about 8-10 feet long. These plates weigh thousands of pounds. The usual procedure for putting the plates into place and removing them involves workmen prying up one edge of the plate, for example with a crowbar, placing a block of wood or the like under the pried up edge, then putting a chain through a hole near the edge and placing the workmen's arm beneath the plate to draw the chain through the hole. Only then can a lifting device such as a crane be employed to put force on the chain thereby lifting the plate.
In order to move the plate it must be lifted the distance approximately its maximum dimension because it hangs from a hole in the edge of the plate. Furthermore, the last portion of the plate to leave the ground is generally not on the center of gravity of the plate, and as the plate leaves the ground, it generally swings back and forth and spins. As a result, movement of cover plates is very dangerous. It is dangerous in a first instance because workmen must pry heavy plates from their supporting surface and must reach beneath the plates in order to feed a chain through the lifting hole. This procedure accounts for many crushed feet and hands. Moving the plates is dangerous in the second instance because the heavy plate with sharp corners swings unpredictably about its supporting chain. These swinging plates account for many lacerations and fractures. Finally, movement of plates is dangerous in the third instance because workmen must guide the swinging, asymmetrically-supported plates into position so that they may be lowered into the desired place, and after the plates are in position, the supporting chain must be removed largely by a reverse of the process with which it is attached.
When not in use covering holes or trenches, the steel plates are usually stacked one on top of another. Although this is an effective and space-saving way to store them, it causes another difficulty because it is so difficult to separate one heavy plate stacked upon another when the contacting surfaces are so flat and in such close contact. The use of a crowbar is difficult because there is not adequate room between the plates to provide a secure prying surface.